When assigning a cover photo to a songbook, you double tap the image in the songbook info mode and such get into the image composer. After you have selected a photo (or a pdf or a combination of those), you must tap the Add Page button before tapping the Done button.

Having to tap both the 'Add Page' and the 'Done' button is misleading in this case, when you want to assign just one cover photo. Though this isn't a real bug, it will be fixed in the next update. [fixed in v2.6]